The use of radars in automotive and industrial applications is evolving rapidly. Radar finds use in number of applications associated with a vehicle such as collision warning, blind spot warning, lane change assist, parking assist and rear collision warning. In industrial environment, radar finds use in determining relative position and relative velocity of obstacles around the radar. Pulse radar and FMCW (Frequency Modulated Continuous Wave) radar are predominately used in such applications.
In an FMCW radar, a synthesizer generates a ramp segment by frequency modulating a transmit signal. The ramp segment is also referred as a chirp signal. The ramp segment is amplified and emitted by one or more transmit units. The ramp segment is scattered by one or more obstacles to generate a scattered signal. The scattered signal is received by one or more receive units in the FMCW radar. A signal obtained by mixing the ramp segment and the scattered signal is termed as an IF (intermediate frequency) signal. The frequency (f) of the IF signal is proportional to the distance (d) of the obstacle from the FMCW radar and also to the slope (S) of the ramp segment.
The IF signal is sampled by an ADC (analog to digital converter). A sampled data generated by the ADC is processed by processor to obtain a position and a velocity of one or more obstacles. In one kind of FMCW radar, the processor performs FFT (fast fourier transform) on the sampled data. A peak in the FFT spectrum represents an obstacle and a location of the peak in the FFT spectrum is proportional to a relative distance of the obstacle from the FMCW radar.
An ideal synthesizer generates a linear ramp segment i.e. frequency varies linearly with time. A corresponding FFT spectrum obtained provides the information of the one or more obstacles. However, practical synthesizer generates a non-linear chirp signal due to finite settling time of the synthesizer. A non-linear chirp signal is one whose frequency does not vary linearly with time. The non-linearity in the chirp signal results in smearing (or overlapping) of peaks in the FFT spectrum which makes the detection of closely spaced obstacles difficult. Further, the smearing of peaks results in ghost objects since a point object will appear as multiple objects. Ghost objects are objects falsely detected by the FMCW radar which are not present in reality. The frequency non-linearity in the chirp signal therefore results in false detection of ghost objects and failure of detection of objects present in reality.